Abnormal eye movements found in CNS diseases can sometimes be revealing about the underlying pathology. Although the oculomotor nuclei, their descending primary motor neurons, and the extraocular muscles they innervate ultimately mediate eye movements, higher centers, including premotor areas in the brainstem, midbrain, subcortical areas, and cerebral cortex, are also involved in the movements. The smooth pursuit eye movement system and the saccadic eye movement system are of interest, for they seem to be abnormal in CNS disorders, where disease in higher subcortical and cortical centers is implicated. Moreover, smooth pursuit and saccadic eye movements have distinct and easily measured properties. Disorders of smooth pursuit eye movements (SPEM) have been found in schizophrenia, CNS illnesses affecting the oculomotor nuclei (e.g., brainstem infarction), diffuse brain diseases (such as Disorders of the Alzheimer's Type), Huntington's disease, and Parkinson's disease. We have designed and built a portable, PC-based system to measure smooth pursuit and saccadic eye movements.